1. Field of the Invention
The present invention relates generally to a control system including a digital computer or microcomputer system being capable of being mounted on an automotive vehicle for controlling various vehicle devices. More particularly, the present invention relates to a method and means for checking elements of the control system, after the control system has been assembled.
2. Background of the Invention
As is known to those skilled in the art, in recent years, it has become quite popular to control various vehicle systems automatically by using control system including a microcomputer unit. The control system is employed in an automotive vehicle for automatically controlling driving of the internal combustion engine, ignition system, exhaust gas recirculation system, fuel injection system and so on. The control system may be further utilized for controlling various vehicle indicators, such as, for example, a navigation meter. Further, the control system is also used for controlling various vehicle equipment, such as an automatic tuning device for a radio receiver of vehicle mounted type.
The control system generally comprises a central processing unit (CPU), an input unit, an output unit and one or more memory units including read-out memory (ROM) and random access memory (RAM). These elements form a microcomputer for processing data inputted thereinto for outputting control signals to the various vehicle devices. For performing control operation, control programs which are generally stored in the ROM and read out and executed in the CPU with respect to input data inputted through the input unit. Result of execution of the control program is outputted to vehicle devices to be controlled through an output unit. In order to satisfactorily control the vehicle devices, correct and accurate function of each element will naturally be required:
the input unit should input signals and read out accurately identical data contained in the input signals;
the RAM should be capable of storing data accurately identical to the data inputted therein and reading out the stored data which should be identical to the data inputted;
the RAM should be capable of keeping storage as preset therein and being read out as is; and
in respect to the output unit, it should be capable of inputting output data inputted from the CPU and outputting the data as is.
For the purpose of checking the control system and for preventing the vehicle from serious trouble caused thereon due to error of the control system and from accidents, checking of functions of the elements of the control system will be required.
The control circuit of the control system determines a change in output signals in response to an input signal or to an elapsed time interval. This function of the control system is substantially the same as that of conventional analog control systems. Therefore, as with conventional analog control circuits, it is possible to check the functions of the control circuit by changing the input signals, and observing the changes in the output signals in response to the input changes, and checking whether a certain relation holds between the observations. However, for the reasons described below the time required for such checking is long, and it is not possible to check all functions perfectly.
(1) It is practice in the conventional analog control circuit to check two extreme values of a function and assume the continuity of the interval between those extreme values. Meanwhile, as in the control system employing a microcomputer therein, binary digital signals are used, and thus, the signals are not continuous and have no relationship with one another. Therefore, it is impossible to perform checking as in the analog case. Now consider the relation between the value of input x and the value of output y in both digital and analog cases for a simple analog function y=0.5x+1.5. In the analog case, for such a simple function the method employing an operational amplifier to give a zero point and a gain is widely used. In this case, the output is checked when the input is minimum, and the output is also checked when the input is maximum. Because the relation is linear in the analog case, the outputs for all the other input values are assured. If the circuit malfunctions one or other of the end values will be in error. In the digital case, a table look-up is widely used as a means for providing such a function. In this method, output data corresponding to input values 0, 1, 2 . . . is stored in advance, and according to the input value the corresponding output value is retrieved and output. In this case, even if the data corresponding to the minimum and maximum output values is correct, the data corresponding to other input values is not assured. It is therefore necessary to check the data for all input values. For this reason, although for conventional analog circuits it was satisfactory to check two values, for digital circuits the number of checking points has to be increased enormously.
(2) Because it is difficult to observe the state of the signals within the control circuit it is not possible to make an accurate check. For example, in the type of case described above where a simple relation holds directly between the input and output, by increasing the number of checking points it is possible to make a check after a fashion, but when the calculation is used at an intermediate point in the circuit a check is not possible. In other words, in the analog case, if test terminals are provided at the input and output points of the circuit, by measuring the voltages on these terminals it is possible to make a check, but in the digital case since a single line is used to indicate many different signals by time slicing, fitting test terminals is of no use, and in practice it is not feasible to detach a single section to test it. Thus the detailed internal operation of the control circuit cannot be tested, and an accurate complete check is not possible. If the circuit is constructed with LSI techniques it may not even be possible to fit checking terminals.
(3) Because a circuit for an automotive vehicle control device is required to be resistant to vibration, it is not possible to construct the circuit using IC sockets and so forth to mount the elements of the circuit. An IC socket is a device to mount an IC on a printed circuit board and thus by using IC sockets, it is possible to remove the IC from the printed circuit board very simply. Thus for devices which do not require any resistance to vibration, by using IC sockets, even after the device is assembled, the ICs can be detached and tested individually with test equipment, but for control devices for use in an automotive vehicle, resistance to vibration is extremely important. For this reason IC sockets, which allow the possibility of unsatisfactory electrical contact caused by vibration, cannot be used. Therefore methods involving the separate testing of elements of the circuit after assembly, are not applicable.
(4) Control circuits for use in automotive vehicles require resistance to water and moisture, and therefore after assembly the surface of the circuit is encased in a waterproof membrane. Therefore checking devices such as logic analysers cannot be used. A logic analyser is a device for observing the movement of many digital signals on a data bus and so forth, and allows the observation of the status of the input and output data of the CPU for example. If a logic analyser is used, the type of control operation described in (2) above can be measured and observed to a certain extent, but when the surface is covered with a waterproof profective membrane of silicone or epoxy resin in order to increase the resistance to water and moisture, such a checking device cannot be connected, and thus cannot be used.
For the above described reasons, for a control device for a motor vehicle which performs digital control on a large scale using for example a microcomputer, testing after assembly takes a long time, and moreover only an unsatisfactory check can be made.
Then, for similar reasons, once the device is handed over to the user, it is not possible to check whether the control circuit is correctly carrying out the actual processes used in the application. Thus, it is not possible to check for faults when carrying out inspection or servicing of the vehicle.
The object of the present invention is to provide a control device for use in an automotive vehicle which, in order to overcome the above described defects of conventional control devices for use in an automotive vehicle, allows a check program different from the normal control program to be connected to the controlling CPU, and starts the check program when certain predetermined conditions occur, so that the control program automatically checks each function of the control circuit and outputs the results, thus allowing the functions to be tested simply and in a short time after the control device is assembled.